Skip tests on Actions if [ci skip]

It seems that we cannot easily apply job-level [ci skip].
Therefore this commit skips only Tests step if it's [ci skip].

.github/workflows/macos.yml

@@ -47,14 +47,14 @@ jobs:
         env:
           TESTOPTS: "$JOBS -q --tty=no"
           MSPECOPT: "-ff" # not using `-j` because sometimes `mspec -j` silently dies
-        if: matrix.test_task != 'test-bundled-gems'
+        if: matrix.test_task != 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       # test-bundled-gems is separated for marking `continue-on-error` because it randomly fails.
       - name: Tests
         run: make -s ${{ matrix.test_task }}
         env:
           TESTOPTS: "$JOBS -q --tty=no"
         continue-on-error: true
-        if: matrix.test_task == 'test-bundled-gems'
+        if: matrix.test_task == 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       - name: Leaked Globals
         run: make -s leaked-globals
       - uses: k0kubun/action-slack@v1.0.0

.github/workflows/ubuntu.yml

@@ -50,14 +50,14 @@ jobs:
         env:
           TESTOPTS: "$JOBS -q --tty=no"
           MSPECOPT: "-ff" # not using `-j` because sometimes `mspec -j` silently dies
-        if: matrix.test_task != 'test-bundled-gems'
+        if: matrix.test_task != 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       # test-bundled-gems is separated for marking `continue-on-error` because it randomly fails.
       - name: Tests
         run: make -s ${{ matrix.test_task }}
         env:
           TESTOPTS: "$JOBS -q --tty=no"
         continue-on-error: true
-        if: matrix.test_task == 'test-bundled-gems'
+        if: matrix.test_task == 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       - name: Leaked Globals
         run: make -s leaked-globals
       - uses: k0kubun/action-slack@v1.0.0

.github/workflows/windows.yml

@@ -53,6 +53,7 @@ jobs:
         run: |
           call "C:\Program Files (x86)\Microsoft Visual Studio\${{ matrix.vs }}\Enterprise\VC\Auxiliary\Build\vcvars64.bat"
           nmake ${{ matrix.test_task }}
+        if: "!contains(github.event.head_commit.message, '[ci skip]')"
       - uses: k0kubun/action-slack@v1.0.0
         with:
           payload: |